There has been no spectacle in the history of televised sport as compelling and atrocious as the night of 29 May 1985, when 39 Italians were killed on the terraces of the Heysel stadium, Brussels, in the murderous prelude to a European Cup final.

It should have been a balmy evening of festival and triumph between Liverpool and Juventus, who were, respectively, then the toughest and most elegant teams in Europe. Liverpool had won the competition the previous year in Rome against the home side AS Roma and now stood poised to take their fifth European Cup.

The Reds had never seemed more imperious. Juventus were perhaps the only side in Europe who could match them. The team of Zbigniew Boniek, Marco Tardelli and Michel Platini was hard-tackling but also one of passion and flair. Platini had vowed to fans to make up for a disappointing season by inspiring Juve to a historic victory.

The stage was set for a classic confrontation. The game was billed as a carnival of attacking football. Instead, it was a scene of carnage. Bodies were piled up at the side of the pitch or just underneath the directors' box. Throughout the game, they were visible to many fans, including players' wives. One of the most distressing sights - and certainly one of the most difficult to forget - was that of a homely, mustachioed Italian fat guy, sprawled with his dead and dying mates in the rubble on the terracing, his hand held aloft in a frozen gesture of despair. This was a scene from a war, not a football match.

Twenty years on this image has lost none of its power to shock. Most witnesses agree that the violence had started early, around 90 minutes before the scheduled kick-off. The stadium was buzzing with songs and skirmishes and an extraordinarily hostile atmosphere between the two sets of fans. The hostility was explained by some Liverpool fans as payback for the final in Rome the previous year, when Liverpool had beaten the home side on penalties. As Liverpool fans had left the stadium after that match they had been pelted with stones and other missiles by the Roma tifosi. No matter that Juve fans had little in common with their compatriots: Liverpudlians were this year in no mood to give ground easily to any Italian fans.

The atmosphere had been noted by the Liverpool players on the pitch before the game and then as they waited in the dressing room. Alan Hansen, a veteran of some of the most difficult grounds in Europe, remembers remarking to left-back Alan Kennedy as they strolled on the pitch that it was the first time that he had seen supporters taking bricks into a stadium. 'They're not throwing bricks,' replied Kennedy. 'They're throwing the stadium at us.'

The most important football match in Europe was being staged in a crumbling ruin. There was already an understandable feeling of unease in the Liverpool dressing room, the result of manager Joe Fagan announcing his retirement the day before the game. The Heysel final would be his last match in charge.

Kennedy, whose goals had been crucial to Liverpool's European success, was out of action that night but travelled with the team to keep up morale. It was his job to keep visiting the pitch to monitor the situation. Every time he reported back to his team-mates, the news was worse. As the violence and the rumours of casualties escalated, Hansen noted how Fagan, whose whole life until this point had been football, began to resemble 'a broken man'.

The most dangerous area of the stadium, clearly visible to Kennedy from the players' tunnel, was the so-called Sector Z, where Liverpool and Juventus supporters stood merely yards apart. They were separated only by a thinly policed no-man's land and temporary chicken-wire fencing. Most of the non-Liverpool fans here were in fact neutrals or expatriate Italians and their children. As the kick-off drew nearer, Liverpool fans began to shower the 'Italian' fans with beer cans, stones, with whatever came to hand. When the 'Italians' responded in kind, the enraged Liverpool fans stormed the no mans' land and the fencing and found, to their surprise, they had free rein.

The fencing was broken down and turned into weapons. The terrified Belgian and Italian fans had no way out and, in desperate frustration, stampeded into a perimeter wall. As it collapsed, killing the 39 innocents and maiming 600 more, the world watched in horror as Liverpool fans turned their attentions to the riot police who were now belatedly entering the stadium. This was the scene that has entered football history as the 'JFK moment': when everybody who was watching remembers where they were and how they felt.

Earlier, British viewers had switched on BBC1 to see Terry Wogan joshing with Bruce Forsyth in the moments before the match was to begin. Suddenly the mood shifted as a harrowed Jimmy Hill introduced a confusing scene of Liverpool fans throwing cans and bottles at the police. The voice of commentator Barry Davies was trembling as he tried to explain with words that continue to move today: 'There are reports that some fans have been killed ... others have been injured.'

In the studio, Graeme Souness and Terry Venables struggled even to say anything at all. As the bodies piled up and the battle raged, I felt, like most people watching, deep shame and anger. The sense of guilt was personal. I was a Liverpool fan. I was from the city. I was one of them. I wasn't at Heysel that night but I had been on my way. I was travelling in France with a party of Liverpool fans who had been delayed on their way to Brussels. We ended up in La Guillotière, the immigrant quarter of Lyon, and drank our way through the afternoon and evening in a bar, which we draped in Liverpool colours.

As the tragedy unfolded on television we watched with mounting horror as we recognised the faces, clothes and style of the lads on the rampage. Nobody could speak. We drank more in the silence. The Arabs in the bar stared at us hard in our Reds' shirts, with our flags and banners, but said little or nothing. I remember most of all watching a girl in a blue jumper trying to climb over the shattered dead bodies, and slipping on the bloody fencing, falling surely to her death. In the days to follow, the media across Europe howled with outrage and with one voice at 'English murderers', 'Assassins' and 'Reds Animals' (this was also a banner produced in the Heysel stadium by Juve fans).

In France, Germany, Italy and elsewhere they laid the blame at our violent culture, the legacy of colonialism, the arrogance and stupidity of the Thatcher government. In Britain itself, the government, the football authorities, the press and the television were stunned. 'It isn't that we're numb,' said Margaret Thatcher. 'We're worse than numb.' For once the tabloid headlines had it right: this was literally, as the Mirror put it, 'the day the football died'.

In the immediate aftermath, there was widespread anger and confusion. In Italy, the British embassy and other national interests were attacked. Across Europe and the world, English working-class males, particularly those from Liverpool, became a byword for ignorance and savagery. In Britain, Thatcher cancelled a cabinet meeting and declared war on 'the hooligan problem' with much the same rhetoric as she had vowed to wipe out the striking miners' unions.

Within days, Liverpool chairman John Smith announced that the club would be withdrawing from the following season's Uefa Cup. Within hours of this announcement, under pressure from Thatcher herself, the Football Association announced that English clubs would be banned from Europe for a year. This was still not enough and two days later Uefa, the game's European governing body, declared that all English clubs were banned from Europe for an indefinite period. In practice this would be five years.

This punishment was followed by the arrest and deportation of Liverpool fans, identified in the press and on television, in what was the largest and most bizarre criminal deportation in British legal history. The Liverpool Echo assumed the role of guardian of the city's conscience and declared 29 May as the blackest day. It reported that Bill Shankly's widow, Nessie, had turned off the match to pray for the victims. In the days to come the letters' page was filled by fans' accounts of the disaster. The predominant emotion was disgust and self-hate mixed with an urgent desire for expiation.

One lad confessed that he had been part of the charge in 'Sector Z' but had broken down weeping when he saw what he had done. There was a cover story about the 'heroic Scouser who saved lives'. In stark contrast, K Martindale of Mossley Hill wrote to the Echo to say he had seen fans pick-pocketing and kicking the corpses. On the ferry from Zeebrugge, disgusted and traumatised, he reported that a grinning lout had boasted: 'We killed forty!'

The truth was that nobody in the city knew how to react. Partly this was because it was so hard to work out what had happened exactly, especially if you had actually been there. Fans' stories varied wildly, laying the blame at, variously, aggressive Juve fans, the cowardly Belgian police, the decrepit stadium. Like many supporters, Steve Kneale, now working for Liverpool City Council, knew nothing of the full extent of the tragedy until he and his mates saw the headlines the following day. 'We were reading the papers in a hotel and everybody around us - French and Belgians - were all silent and didn't want to speak to us,' he told me recently in Liverpool.

'I remember one of the lads, an older feller who'd travelled most with Liverpool, saying quietly that we should just go home. We went out into the street and this old woman spat at us and started shouting in Flemish.' The Echo devoted swaths of space to witnesses' accounts reporting that the riot had been the work of the National Front, Chelsea or Leeds supporters or had even, as one diehard Red insisted, been provoked by disgruntled Evertonians. Liberal councillor Peter Millea, who was at the match, told the Echo he distinctly made out a contingent of skinheads wearing Union Jack T-shirts and speaking with cockney accents.

'No Scousers dress like that ,' he said, 'and it was clear they had just come for trouble.' Liverpool chairman Smith issued a statement saying that the troublemakers were NF supporters, probably from London, and that a shot had been fired before the worst part of the riot.

Fans desperately wanted to believe that all of this was true. Most of these rumours have long since been dismissed as fabrications. John Williams, of the Sir Norman Chester Centre for Football Research at Leicester University, is however still unsure about what actually took place on the terrace. Williams is probably the foremost authority on football culture in the Seventies and Eighties, and is a lifelong Liverpool fan. It was his direct knowledge of Anfield that made him sceptical then and now about the official accounts of what happened.

'None of these rumours about National Front supporters was ever proved, or ever will be proved,' he says, 'and it is true that these urban myths are a neat way of avoiding reality. But to anybody at the time who knew the culture of Liverpool FC and its supporters, there was something that rang false in all the media accounts. The fact remains that Heysel was totally alien to Liverpool football culture.'

Williams argues that Liverpool terrace culture was at odds with the kind of hooliganism that blighted the game from the mid-Seventies onwards. 'There were often fights at matches,' he says , 'but in general Liverpool fans were not really interested in violence, although they could look after themselves. The idea of being a "scally" was to be above the kind of pointless destruction which neanderthals like Leeds or Chelsea or England fans went in for. That is why Liverpool fans were never organised into firms; there were just small bunches of mates who stood together at the match or in the pub. It was okay to steal - as long as what you stole was designer stuff from Europe or wherever. It was just funny stuff, larking about.'

A similar ethos was promoted in The End, the most popular Anfield fanzine of the era. Funnier than Viz and sharper than the Face, it was firmly rooted in a scally culture of robbing, smoking 'draw' and shagging. The fanzine was celebrated by John Peel, among others, who wore a The End T-shirt on Top of the Pops, and in the pages of the NME and Sounds. Its editors (who included Peter Hooton, who would later find fame with the band the Farm) mocked 'agriculturals' and 'woollybacks' from Manchester and Leeds (the striking miners were the one exemption to this code on the grounds of their attacks on the southern establishment).

They laughed long and loudest at Cockneys (and particularly Chelsea fans) for their stupid accents and lousy clothes. If this was a fiercely parochial view of the world, it was not especially violent ('All football hooligans are dickheads,' was a common refrain on the letters' page). The End was, in its own way, the heir to the irreverent and clever Liverpool tradition of the 1960s, which produced the Beatles and a whole generation of artists, comics and writers.

But this was not, by 1985, how the rest of England saw Liverpool . By then, the city was noted for industrial decline and social unrest. In a country changing to keep pace with the hucksterism of the Thatcher counter-revolution, Liverpool, with its unemployment, riots and far-left politics, seemed a strange and separate place. Peter Hooton was at Heysel. He is honest enough to concede that Liverpool fans were responsible for what happened.

'I'd never say they weren't Liverpool fans because they obviously were,' he says now. 'There was none of this mythical "organisations" from London or other people.'

The shock of Heysel for Hooton and others was that it confirmed the outside world's view of Liverpool. Most significantly and damagingly, it revealed a dark side to the city's culture to those who thought they knew it best. This is one key reason why Heysel remains largely a taboo subject in Liverpool.

A second is that feelings over Heysel are also related to the Hillsborough disaster of 1989, when 95 Liverpool fans were crushed to death, with another dying later. Then, Liverpool fans were the victims. This did not make Hillsborough any less traumatic, but it did mean that it was easier to come to terms with. As one witness to both Hillsborough and Heysel put it: 'It is the difference between a bereavement and a murder.'

One of the most extraordinary and, for many, still shameful facts about Heysel is that the match actually took place. The accepted truth is to say that if it hadn't there would have been a bloodbath in the streets. Nobody can say at this distance whether this would have been the case, but given the disgracefully bad policing, it may well have happened. Another accepted truism is to say that when the kick-off took place everybody had forgotten the football. But this wasn't strictly true, not for thousands of fans in the stadium, and not where I was standing at the time.

Certainly I was not alone in the crowd of fans in a bar in France in willing Liverpool to win. We complained loud and long about the phoney penalty, scored by Michel Platini, which won Juventus the Cup. As the match finished, a window in the cafe was smashed. We walked out of the bar to face a small mob of locals. We tore into the pack. We weren't psychopaths or even normally football hooligans. But we fought as hard we could.

Even 20 years on, I find it hard to explain this. How could anyone justify more violence in the immediate wake of a televised massacre? The simple reason is that we refused to be ashamed of who we were. Many of those in Heysel that night have told me that they felt the same emotion. Guilt was inextricably linked to defiance, which in turn created more violence. One of the most penetrating studies of these emotions is to be found in the 1991 book Amongst the Thugs by Bill Buford, which was written explicitly as a response to Heysel.

Most devastating of all is Buford's firsthand account of the excitement of terrace violence, the electricity that sweeps through a mob, turning it into a murder machine. He quotes Susan Sontag and Georges Bataille to explain that the real attraction of hooliganism is to be found in the 'elation of transgression', a 'druggy high' when everything is perceived in 'slow motion and great detail'.

Fascinatingly, Buford - an educated middle-class American, a stranger to football culture - is drawn in and enjoys the violence he comes to report on. My own final memories of Heysel are, in the same way, not just shame but also, at some deep and untouchable level, the exhilaration of combat. All football fans understand this, viscerally. In Fever Pitch, Nick Hornby puts it in simple but extremely effective terms:

'The kids' stuff that proved murderous in Brussels belonged firmly and clearly on a continuum of apparently harmless but obviously threatening acts - violent chants, wanker signs, the whole, petty hardact works - in which a very large minority of fans had been indulging for nearly 20 years. In short Heysel was an organic part of a culture that many of us, myself included, had contributed towards.'

Heysel has inspired few cultural responses. Liverpool playwrights Willy Russell and Alan Bleasdale were at their prolific best during this period, but produced no major work that dealt with the subject. One low-key response from high culture was 'Memorial' by Michael Nyman. This elegiac piece was performed once only, in a warehouse in Rouen, days after the disaster. It was described in the Guardian by Waldemar Januszczak as 'a small piece of atonement'. It resurfaced as part of the soundtrack to the Peter Greenaway film The Cook, The Thief, His Wife and Her Lover but has been forgotten or ignored by those who were at Heysel.

Appropriately, Nyman's work, intended as a homage to the dead, has indeed had a greater impact in Europe, where one Italian critic described it as an 'inexpressibly sad and epic funeral march'. European commentators on Heysel have seldom engaged with, let alone understood, the intricacies and nuances of English football cultures. However, a few recent works have provided useful insights.

One of the most powerful and moving is a 2003 book from Italy called Le verità sull'Heysel ('The truths on Heysel') by the veteran sports journalist Francesco Caremani. This is an attempt to understand the violence from the point of view of those who are attacked and killed. This does not make it a book written against Liverpool fans or Liverpool FC, but rather, as Caremani himself says, 'a book about human motives and suffering'.

Most important, and most bravely for a writer writing from the Italian point of view, Caremani does not flinch from describing all those present at Heysel as victims in the sense that they were playing roles in a larger tragedy that they did and could not understand at the time. This is the perspective of the French media theorist Jean Baudrillard, who devotes a chapter in his book The Transparency of Evil to Heysel.

His analysis is uncharacteristically straightforward and clear-eyed. He says Heysel was a primitive but devastatingly effective form of 'interactive television'. He points the finger at the Thatcher government's war with the miners (which he describes as 'state terrorism'), which he says was bound to lead directly or indirectly to eruptions of violence at sporting 'pseudo-events'.

Heysel, says Baudrillard, did not happen by chance; it was the inevitable result of the desire of spectators to turn themselves into actors. The nature of the violence itself - crude, tribal and pointless - was a cultural reflex conditioned by circumstance and environment. For once Baudrillard is not overstating the case for effect. It is easy to forget what a violent and unstable place Britain was in the early 1980s and how poor the conditions were for football fans during this period.

Since the early 1970s English fans had been wreaking havoc in Europe and, at home, on each other. Their behaviour was received with platitudes and inertia from the media and the government. Those who ran the game, those who could do something about the bad grounds, the lousy security, the climate of hate and the racism, invariably looked away. Everybody who attended a match during this period knew that something was deeply wrong.

Heysel changed everything about the culture of English football, much of it ultimately for the better. Its antithesis was probably Italia 90 when, in a fog of spliffed-out bliss and to the soundtrack of New Order, English football began its long renaissance. Since then we've moved on to the age of the 'post-fan', the age of consumers rather than supporters.

Even so, no one these days can seriously wish for a return to the bad old days before the Taylor report, the judicial inquiry commissioned in the wake of Hillsborough that led to widespread reform, including all-seat stadiums and intelligent policing. And yet there remains a strong sense in Liverpool, England and Europe that what happened at Heysel is unfinished business.

For a variety of reasons, it has not been quite expunged from memory. Perhaps it never will be. The hard line of Uefa has had far-reaching consequences for the game in England and there are still those (such as Howard Kendall, then manager of Everton, who had won the Cup Winners' Cup that month and stood poised for the Champions Cup in the following season) who talk regretfully about lost opportunities and a lost generation of English players.

They blame Liverpool fans for destroying a whole era in football history and for ending the dominance of English clubs. The truth is that the collapsed wall at Heysel was a deadly metaphor for the gathering destructive forces that brought English football culture to its knees. Most significantly, Heysel marked the culmination of a long trajectory of violence and neglect in England's football culture, which, despite the success of its clubs in Europe, was heading inexorably for self-destruction.

Looking back, it is a miracle that anyone has made it out of the wreckage.

· Andrew Hussey is a contributing editor of Observer Sport Monthly. He is the author of The Game of War: The Life and Death of Guy Debord (Pimlico). He is writing a history of cultural subversion in Paris to be published in Penguin in 2006. His interview-profile of Zinedine Zidane was published in our issue of April 2004